Preheat oven to 350°F (175°C). Grease a 9x13 inch baking pan.
In a large mixing bowl, whisk together flour, sugar, baking powder, and salt.
Stir in milk, eggs, vanilla extract, and melted butter until well combined. Pour batter into prepared pan.
In a separate bowl, mix together softened butter, brown sugar, flour, and cinnamon until well combined. Drop spoonfuls of the cinnamon mixture over the batter and use a knife to swirl it through the batter.
Bake for 30 minutes or until a toothpick inserted into the center comes out clean. Allow to cool slightly before serving.